For the next expansion: skill trees and/or more varied gearset options.

[Alien_Gamer]

There will be one build superior to the others and this will become mandatory. Anyone not using this build will be treated badly by some and it will create more division in the player base.
Devs will have an even harder time balancing and eventually just give up; leaving a clear and zero-choice route to the optimal build.
This is just the way of it in class-based MMOs.

While you aren't wrong, there is room for it within FF14 using the Role abilities.

For example, they could add more defensive CDs and oGCD attacks for MT and OT. Healers could spec between buffs, regen or shield mitigation depending on playstyle and party comp. DPS could spec between solo buffs, party buffs or extra oGCD attacks.

Depending on if you're soloing or what kind of group content you're doing there would be a different set of skills you'd want.